Need help interpreting chrono data

[AMMASHOOTA]

I was out at the range today, continuing to work up a load for my new .308 (Krieger 26” 11 twist on a Rem. 700 action)

I started out with really conservative loads and currently am around the middle of the range for the powders I have been working with. It is the first time I have set up my chrono to have a look-see what the loads were actually doing, and the results have left me a bit perplexed.

I have been using load data from the Vihta Vouri manual (3rd edition). I started working with N150 because I have an 8 pound jug of it. I know that this is not considered to be the best powder for .308, but I figured I’d try it since I had so much of it around.

A couple of weeks ago I was shooting at 85 degrees, and my groups were nice and tight (1/2 inch and less). Today the temp dropped to 60 degrees, and the results were not as good. My groups opened up considerably, even though the loads were about 1-2 grains higher than previous loads.

First, a couple of basics: I am very diligent in my loading technique. All loads are weighed on an RCBS charge master 1500, and I am absolutely certain the weights were both accurate and consistent. I was using 150 grain Sierra Match Kings, loaded to 2.800.

Here are the results from the chrony:

45.3 grains
2849, 2851, 2849, 2836, 2833

45.4 grains
2798, 2807, 2821, 2802, 2820

45.5 grains
2883, 2884, 2857, 2866

Question #1. How can a 1 grain bump results in a decrease in speed between 45.3 and 45.4 grains? (I am absolutely certain the charge weights were accurate)

Question #2. The VV manual lists 47.0 grains (2767 fps) as the max load. How can I have exceeded this speed with such a light load?

None of the 20 rounds I shot (I also shot 5 rounds at 45.2 grains, but did not have the chrony set up for that group) showed any pressure signs at all.
All 5 shot groups were done on a cold barrel.

Being somewhat disappointed with the results, I let the barrel cool and shot one last group with the same bullet and 43.0 grains of N140. These results were spectacular. A 3 shot group measured 0.308, while not world class, at least what I consider to be quite acceptable for shooting from a bipod.

43.0 grains of N140
2754, 2751, 2770

Again, these results don’t correlate with the manual, which lists 45.8 grains (2761 fps) as a max load. These rounds also showed no pressure signs.

Question #3. Can I continue to increase this last load, since it appears to be well under the 45.8 grain limit, even though my speed is very close to what is listed as a max load?

I realize that temperature can have a dramatic affect on load development. I am assuming that my pressures were much higher a couple of weeks ago in the higher ambient temperature, resulting in smaller group size. I also know that values in the manual may not adequately reflect the conditions i am shooting under, but since my results are so different from what is listed, I am a little leery of which direction to proceed in.

I probably will stick with the N140 for the time being, since it is giving me much better results than the N150.

 

[AMMASHOOTA]

do you load at the range?

no

 

[AMMASHOOTA]

bummer


I've got a whole regimen for working up to max, mapping velocities etc but you must load/shoot for it to work.

My advice, just stay safe/low if you must preload.

al

i can adapt, it will just take a while. our range is fairly comprehensive, but more work (and thought) was put into the pistol bays than the rifle range. our shooting tables were poorly designed. i am starting to redesign one of them to make it a bit more practical. once i have done that it may be suitable for loading at the range.

money is always an issue. i love this sport, but i sink sooo much of my free cash into so much stuff that there isn't a lot left to go around when it comes to thinking about an arbor press and a set of wilson dies for loading out there.

i'm mainly a pistol shooter who has recently gotten back into rifle shooting and am having a great time with it. i am splitting my time between training, competing, and shooting my .308. there are just not enough hours in the day or energy in this 62 year old body to accomplish everything.

safety is of course my primary concern, so when i see an inconsistency in my loading, i take a step back and ask the experts here for help.

i enjoy the discipline that the rifle imposes on you, and have become a whole lot better in the past few months than i have ever been before. this of course is an endless circle, for the better you get the better you want to be.

i can see a day in the not to distant future when i will have to stop competing in handgun competitions. when that arrives, i want to be ready to shoot rifle full time.

i appreciate your help.

 

[Cheechako]

[QUOTE=AMMASHOOTA;651980 . . .Question #1. How can a 1 grain bump results in a decrease in speed between 45.3 and 45.4 grains? (I am absolutely certain the charge weights were accurate)

Question #2. The VV manual lists 47.0 grains (2767 fps) as the max load. How can I have exceeded this speed with such a light load? . . .

I need some clarification.

1) The loads you listed showed a difference of only 2/10 of a grain between the high and low, not 2 grains. An ES of 86 fps would not suprise me.

2) Manuals are only intended to be quides. Your rifle/bullet/powder could give a result far different from a manual.

Ray

 

[Charles E]

Jim Borden did a study on what it takes to make preloading work. You have to load to the same velocity, tested as you load. IIRC correctly, he took rounds loaded on three separate days, with a different charge, but all to the same velocity on the day loaded, and shot groups. The groups were very small. Again as I recall, the charge could vary as much as 1.5 grains to achieve the same velocity on different days. Don't remember if this was a Hunter rifle (approx 38 or so grains charge) or a PPC (approx 30 grains charge), but it was that order of magnitude.

Al might remember better, or even have the text stashed away somewhere.

And that would explain what you're seeing.

 

[AMMASHOOTA]

that was a typo on my part. they were 0.1 grain bumps.

 

[Andy Cross]

You will find the maximum

When working up your load just keep adding powder until the first signs of reaching max are found. Note the ambient temperature also. Back off from there until you find the best shooting group. If it is close to maximum I would retest on a day that was hotter. For years I had a winter and summer load for the 6ppc BR rifle. The temps in Oz could range from a pleasant 18degC during winter to 40degC on a summers day.

The load difference for each was about 6%. The grouping ability when shooting a summer load on a cool day was significant. If you have the opportunity try WW748 ball powder in the 308. You might be surprised by the velocity and hopefully the groups.

Andy.

 

[Al Nyhus]

A couple thing to keep in mind, but first:

And you are using windlflags of some sort, right?

- Chronographs have an error factor, normally in the range of 1%.
- Powders that are borderline slow for the application often give results like you're seeing with the N150.

I don't diddle in .2 gr. increments when doing load work with a 308-sized case. Go up in .5 gr. increments and then you can split hairs, if so inclined. Picking up the powder speed (WW748, N140, Varget, etc.) will doubtless net you good results.

Loading at the range is really the way to go and can be as simple or as complex as you want to make it. Size and prep your cases at home. Bolt your home press to a block of wood and secure it with a couple of 'c' clamps to the bench top, tailgate of your pickup...anywhere. Powder can be weighed at home and stored in vials, film canisters, etc. Heck, I used to put charges in Dixie cups when I started loading at the range. Not loading at the range can be very limiting to what you can try, not to mention time and money wasted.

Let us know what you find, okay?

Good shootin'. -Al

 

[AMMASHOOTA]

A couple thing to keep in mind, but first:

And you are using windlflags of some sort, right?

- Chronographs have an error factor, normally in the range of 1%.
- Powders that are borderline slow for the application often give results like you're seeing with the N150.

I don't diddle in .2 gr. increments when doing load work with a 308-sized case. Go up in .5 gr. increments and then you can split hairs, if so inclined. Picking up the powder speed (WW748, N140, Varget, etc.) will doubtless net you good results.

Loading at the range is really the way to go and can be as simple or as complex as you want to make it. Size and prep your cases at home. Bolt your home press to a block of wood and secure it with a couple of 'c' clamps to the bench top, tailgate of your pickup...anywhere. Powder can be weighed at home and stored in vials, film canisters, etc. Heck, I used to put charges in Dixie cups when I started loading at the range. Not loading at the range can be very limiting to what you can try, not to mention time and money wasted.

Let us know what you find, okay?

Good shootin'. -Al

i appreciate the advice. i started with .5g increments and at 85-90 degrees hit a sweet spot at 45.2g so i thought i would see what would happen from there. that's how i got to this thread.

switching from N150 to N140 proved to be a good move given the poor results i was having when the temp dropped 25 degrees.

we only have two wind conditions here: dead calm and a 15mph 0 value wind. i did my initial load development during the windy conditions and my groups averaged around 0.300. i do have flags but i would need to shoot somewhere else to use them.

this barrel does not like Varget -- i tried that first. it does fine with Benchmark, 4064, etc. I will pick up some 748 and give that a try.

 

[mike in co]

there is no way to be sure of these loads with that scale..PERIOD
THE SCALE IS PLUS OR MINUS .1....YOUR LOADS ARE ALL .1 APART.......SILLY TEST/WASTE OF POWDER WITH THE SCALE YOU USED.
mike in co

I was out at the range today, continuing to work up a load for my new .308 (Krieger 26” 11 twist on a Rem. 700 action)

I started out with really conservative loads and currently am around the middle of the range for the powders I have been working with. It is the first time I have set up my chrono to have a look-see what the loads were actually doing, and the results have left me a bit perplexed.

I have been using load data from the Vihta Vouri manual (3rd edition). I started working with N150 because I have an 8 pound jug of it. I know that this is not considered to be the best powder for .308, but I figured I’d try it since I had so much of it around.

A couple of weeks ago I was shooting at 85 degrees, and my groups were nice and tight (1/2 inch and less). Today the temp dropped to 60 degrees, and the results were not as good. My groups opened up considerably, even though the loads were about 1-2 grains higher than previous loads.

First, a couple of basics: I am very diligent in my loading technique. All loads are weighed on an RCBS charge master 1500, and I am absolutely certain the weights were both accurate and consistent. I was using 150 grain Sierra Match Kings, loaded to 2.800.

Here are the results from the chrony:

45.3 grains
2849, 2851, 2849, 2836, 2833

45.4 grains
2798, 2807, 2821, 2802, 2820

45.5 grains
2883, 2884, 2857, 2866

Question #1. How can a 1 grain bump results in a decrease in speed between 45.3 and 45.4 grains? (I am absolutely certain the charge weights were accurate)

Question #2. The VV manual lists 47.0 grains (2767 fps) as the max load. How can I have exceeded this speed with such a light load?

None of the 20 rounds I shot (I also shot 5 rounds at 45.2 grains, but did not have the chrony set up for that group) showed any pressure signs at all.
All 5 shot groups were done on a cold barrel.

Being somewhat disappointed with the results, I let the barrel cool and shot one last group with the same bullet and 43.0 grains of N140. These results were spectacular. A 3 shot group measured 0.308, while not world class, at least what I consider to be quite acceptable for shooting from a bipod.

43.0 grains of N140
2754, 2751, 2770

Again, these results don’t correlate with the manual, which lists 45.8 grains (2761 fps) as a max load. These rounds also showed no pressure signs.

Question #3. Can I continue to increase this last load, since it appears to be well under the 45.8 grain limit, even though my speed is very close to what is listed as a max load?

I realize that temperature can have a dramatic affect on load development. I am assuming that my pressures were much higher a couple of weeks ago in the higher ambient temperature, resulting in smaller group size. I also know that values in the manual may not adequately reflect the conditions i am shooting under, but since my results are so different from what is listed, I am a little leery of which direction to proceed in.

I probably will stick with the N140 for the time being, since it is giving me much better results than the N150.

 

[AMMASHOOTA]

there is no way to be sure of these loads with that scale..PERIOD
THE SCALE IS PLUS OR MINUS .1....YOUR LOADS ARE ALL .1 APART.......SILLY TEST/WASTE OF POWDER WITH THE SCALE YOU USED.
mike in co

pardon me all to hell for running silly tests with *my* powder. please send me a pound or two of what you think i should be shooting and i will be sure to follow whatever directions you enclose with it.

 

[mike in co]

read what i said...it is the claim of three loads being in .1 steps that is the error of the test. the SCALE is not accurate enough to make the claim that they are ACCURATE .1 steps.
no, the powder was not a good choice for the bullet weight, but it is the CLAIM OF ACCURATE .1 STEPS THAT IS THE ERROR OF THE TEST.


the SCALE is plus or minus .1......
so load one is not 45.3 but 45.2 to 45.4
load two is 45.3 to 45.5
and load three is 45.4 to 45.6........meaning it is possible that they were all
45.4......
or any combination of the weight spread.
my reccomendation is NOT TRY .1 steps in a 308 case......and when you want to try small step to get a better scale than one that is plus or minus .1.....that is a .2 spread so no testing below .3....cause the scale can't handle it.....

mike in co

pardon me all to hell for running silly tests with *my* powder. please send me a pound or two of what you think i should be shooting and i will be sure to follow whatever directions you enclose with it.

 

[AMMASHOOTA]

read what i said...it is the claim of three loads being in .1 steps that is the error of the test. the SCALE is not accurate enough to make the claim that they are ACCURATE .1 steps.
no, the powder was not a good choice for the bullet weight, but it is the CLAIM OF ACCURATE .1 STEPS THAT IS THE ERROR OF THE TEST.


the SCALE is plus or minus .1......
so load one is not 45.3 but 45.2 to 45.4
load two is 45.3 to 45.5
and load three is 45.4 to 45.6........meaning it is possible that they were all
45.4......
or any combination of the weight spread.
my reccomendation is NOT TRY .1 steps in a 308 case......and when you want to try small step to get a better scale than one that is plus or minus .1.....that is a .2 spread so no testing below .3....cause the scale can't handle it.....

mike in co

point taken.

 

[Larry Elliott]

I'd say that 0.1 gr changes in charge in a case the size of the .308 even if the charges are weighed to ±0.01 gr on a lab grade scale aren't going to show much change in velocity or accuracy. Changes of 0.2 or 0.3 gr for a case that size are only ~0.5% of the total charge weight which might be considered insignificant. I've noted fairly large variations with the same load when chronographing on different days even when temperatures were practically the same. Distance from muzzle to the center of the screens different, screens not aligned the same way from one session to the next, sun at a different angle or clouds different, and a whole bunch of causes I'm probably not bright enough to understand or consider. But there are a lot of things to learn or try to learn when developing loads and chronographing.

As others have said manuals aren't the perfect answer, they give an idea. If the test barrel VV used was shorter than your barrel, the bore was tighter or looser, and differences in chambers could all account for the velocity you're getting compared with the manuals for N140. Also powder lots can and do produce different velocities and pressures and the cases, primers, and powder lots were unlikely to be the same as used for the manual, not to mention bullets, but they're likely less of an influence from lot to lot of the same brand and type.

 

[AMMASHOOTA]

I'd say that 0.1 gr changes in charge in a case the size of the .308 even if the charges are weighed to ±0.01 gr on a lab grade scale aren't going to show much change in velocity or accuracy. Changes of 0.2 or 0.3 gr for a case that size are only ~0.5% of the total charge weight which might be considered insignificant. I've noted fairly large variations with the same load when chronographing on different days even when temperatures were practically the same. Distance from muzzle to the center of the screens different, screens not aligned the same way from one session to the next, sun at a different angle or clouds different, and a whole bunch of causes I'm probably not bright enough to understand or consider. But there are a lot of things to learn or try to learn when developing loads and chronographing.

As others have said manuals aren't the perfect answer, they give an idea. If the test barrel VV used was shorter than your barrel, the bore was tighter or looser, and differences in chambers could all account for the velocity you're getting compared with the manuals for N140. Also powder lots can and do produce different velocities and pressures and the cases, primers, and powder lots were unlikely to be the same as used for the manual, not to mention bullets, but they're likely less of an influence from lot to lot of the same brand and type.

what i am trying to take away from this is what i should depend on for finding a safe, but effective load.

since i have not started to see any pressure signs yet, i am still a bit perplexed why my loads are above the speed indicated for a max load by a significant factor. i could understand if i was within 0.5g of the manual (or website, etc), but to be over 1.5g short of max and have more velocity than max gives me pause.

i probably picked the wrong time of the year to begin load developement -- between changing seasons, but it is what it is.

the conventional wisdom appears to be to keep pushing until pressure signs appear and then back of around 0.5g

certainly the change from VV 150 to 140 has brought both the velocity and load density much closer to what i would expect under these conditions. i would certainly like to understand why borderline slow powders exhibit such a wide range of behavior.

i know it is apples to oranges, but i use rather slow powders in all of my pistol loads with great success, and without the variation i am seeing here.

i realize that the selecting the fastest powder that gives the best results is the direction to take. i'm just trying to understand the relationship between powder speeds and the other factors that influence a given load.

 

[mike in co]

my opinion is that powders "like" to burn in a certain pressure band. when below the band do not count on anything being "normal".
do i know all the pressure bands..no
simple things like your bbl is 26" and thier test bbl is only 24', or that thier bbl is 1/12 and yours is 1/11, that they shoot indoors in a lab and you don't/
it all matters......thier brass was most likely lapua.....and who's primers where you using ?

you sort of have it right...tune for your bbl.......approach it slowly, and watch what you do and its results....

i shoot a 135 from a 308win BENCHREST at 2900 fps with n140 45gr...24" bbl

 

[AMMASHOOTA]

Just as a follow up: i shot a 0.217@100 yards this weekend with 42.5g of VV N140 and 155g Hornady A-Max bullets. ES was 20 and velocity was 2703. I realize that's a little on the low side, and i will see what bumping it up does, but things have settled down and i am now getting very consistent results.

just wanted to thank everyone for their advice.